1. Field of the Invention
The present invention relates to a data generation apparatus, a printing apparatus and a data generation method. Particularly, the present invention relates to processing for generating print data which is used for completing printing of a unit area of a printing medium by performing a plurality of times of scans (movements) of a print head and by performing conveying of the printing medium between respective the scans (movements).
2. Description of the Related Arts
With the spread of information processing apparatuses such as personal computers, printing apparatuses that serve as image forming terminals have also spread. Inkjet printing apparatuses especially, for performing printing on a print medium such as paper, by ejecting ink onto the medium, through ejection ports, have the included advantages of being non-impact, reduced noise printing types, for which fast printing at high densities are enabled, and of being easily compatible with color printing. Because of these advantages, inkjet printing apparatuses are rapidly becoming the favored, personal use printing apparatuses.
A so-called serial type inkjet printing apparatus frequently employs a multi-pass printing method. It should be noted that “pass” and “scan” used below have the same definition. According to the multi-pass printing method, image data for a predetermined area (unit area) is divided into data for each color and each pass, and a mask is generally employed for this data division.
FIG. 1 is a diagram for explaining a multi-pass printing that employs a mask, and illustrates a print head and printed dot patterns schematically for a case four scans are performed to complete printing of an image in a unit area. In FIG. 1, a reference sign P0001 denotes a print head. To simplify the drawing and the explanation, the print head P0001 is shown to include a nozzle array of 16 ejection ports (hereinafter also referred to as nozzles). The nozzle array is divided into the first to the fourth nozzle groups, each of which includes four nozzles. A reference sign P0002 denotes a mask pattern. In the mask patterns P0002, mask pixels (print permitting pixels) that permit printing correspondingly to the individual nozzles are indicated by solid black. The mask patterns corresponding to the four nozzle groups are complementary to one another, and by superposing the four patterns, all of the 4×4 pixels become print permitting pixels. That is, the four patterns are employed to complete printing of the 4×4 area.
Reference signs P0003 to P0006 denote arrangement patterns of formed dots, which illustrate how an image is completed by repeating printing scans. As shown in these patterns, multi-pass printing forms dots based on binary print data (dot data), which are generated by mask patterns correspondingly to the nozzle groups, in each of printing scans. Then each time a printing scan is completed, the printing medium is conveyed a distance, equivalent to the width of a nozzle group, in the direction indicated by an arrow in FIG. 1. By scanning four times in this manner, an image is formed in the area of the printing medium corresponding to the width of each nozzle group.
According to the above described multi-pass printing method, uneven print densities, which results from a variation in the ink ejection directions and ink ejection amount among multiple nozzles which are due to manufacturing processes of the print head and from an error in paper conveying operation performed between the printing scans, can be made less noticeable.
The example in FIG. 1 is for four-pass printing. The same process is performed for two-pass printing, in which two scans are employed to complete the printing of an image, three-pass printing, in which three scans are employed to complete the printing of an image, or multi-pass printing, in which five or more passes are provided for five or more scans employed to complete the printing of an image. That is, the numbers of ejection port groups provided by dividing ejection ports on a print head and the amount a printing medium is conveyed, which are basically explained while referring to FIG. 1, are determined in accordance with the number of passes for completing printing.
Binary data (dot data) used for multi-pass printing is generated by employing a pseudo gradation method, such as a density pattern method or a dither method. When the density pattern method is employed, several types of density patterns having fixed dot arrangements are provided correspondingly to respective density levels. Then, a density pattern corresponding to an input density level is selected in accordance with a density pattern selection matrix and thereby binary data is generated. When the dither method is employed, binary data is generated using a dither pattern wherein threshold values are arranged in a predetermined pattern.
The density pattern selection matrix and the dither pattern are not prepared as a pattern or matrix of the same size to that of binary data that are expanded, but the density pattern selection matrix and the dither pattern having a predetermined size are repetitively used in accordance with the overall size of the binary data to be expanded.
Conventionally, each of the repetitively used density pattern selection matrix and dither patterns is a single pattern size of which is fixed. According to Japanese Patent Laid-Open No. 2001-54956, a single pattern having a fixed size is employed as a density pattern selection matrix (an index pattern) to perform multi-pass printing. As described in this conventional example, when, for example, the number of passes for multi-pass printing is to be changed according to switching printing modes, the conventional system that performs the multi-pass printing employs a binarization pattern having a fixed size.
However, when a conventional binarization pattern (the density pattern selection matrix, the dither pattern) having a fixed size is universally employed for a multi-pass printing system in which the number of passes is variable, following problems have arisen. More specifically, since a pattern of fixed size may not be appropriate relative to the amount of distance which a printing medium is conveyed between respective scans (hereinafter this amount is also called a feeding amount), an image printing objective designed by the pattern, such as the printing quality, may not be achieved.
This problem is described below specifically. Binary data that is generated based on the density pattern selection matrix or the dither pattern is generated in the cycle of repetition according to the size of a density pattern selection matrix or a dither pattern.
In this case, when the feeding amount is integer multiple of the cycle of repetition of binary data generation (the binarization pattern), the same repetition cycle for binary data is applied for all unit areas. Therefore, the deterioration of image quality does not occur due to a difference in the dot arrangements applied for the unit areas and a difference in the order of dot formation applied for the unit areas. However, when the feeding amount is not an integer multiple of the repetition cycle for binary data (the binarization pattern), the repetition cycle for the binary data appears in a different way among the unit areas. Further, the same repetition cycle for the binary data generation corresponds to two adjacent unit areas, and therefore the binarization pattern that is determined by taking into consideration the image quality objective, especially the size of the binarization pattern, operates over different unit areas. As a result, the dot arrangement and the dot forming order in a unit area may be made different among the unit areas, and deterioration of the image quality may occur that is due to differences in the dot arrangements and in the dot forming order.